This invention relates to a multiple wavelength, surface emitting laser and, more particularly, to a multiple wavelength surface emitting laser with broad bandwidth Distributed Bragg Reflectors (DBRs) to reflect beams of the multiple wavelengths.
Addressable monolithic multiple wavelength light sources, such as laser arrays that can simultaneously emit light beams of different wavelengths from different elements in the array, are useful in a variety of applications, such as color printing, full color digital film recording, color displays, and other optical recording system applications.
Individually, semiconductor light emitting diodes and semiconductor lasers may have insufficient power for some of those applications. Arrays of semiconductor light emitting diodes or semiconductor lasers can be used to increase the total power output, to provide parallel processing, and to simplify optical system design. Arrays have been fabricated so that the light emitting elements are on a single semiconductor substrate in a monolithic structure to provide and maintain good optical alignment of the light emitting elements of the array with one another and to minimize the assembly involved.
One problem with such arrays is maintaining the electrical and optical isolation between the individual light emitting elements. Another problem is increasing the density of the light emitting elements in the substrate by closely spacing the elements together while still maintaining the isolation, avoiding heat dissipation problems, and providing precise alignment of the elements.
Yet another problem is making each individual light emitting element in the array independently addressable. As the light emitting elements are spaced closer together in higher densities, it is progressively more difficult to separately, individually and independently cause each element to emit light.
Also working against the high density is the requirement that the individual closely spaced emitted light beams be easy to separate or detect. Typically, beam separation is accomplished by having the array simultaneously emit light beams of different wavelengths from different elements in the array.
A dual wavelength laser has two active layers for emitting light at two different wavelengths in U.S. Pat. No. 5,708,674, commonly assigned as the present application and herein incorporated by reference. Impurity induced layer intermixing removes a portion of the narrower bandgap, longer wavelength active layer to allow the wider bandgap, shorter wavelength active layer to emit light. However, the dual wavelength laser of that invention is an edge emitter.
Semiconductor lasers are typically edge emitters. The light is emitted from the edge of a monolithic structure of semiconductor layers. An alternative construction is surface emitters where the light is emitted from the surface of the monolithic structure of semiconductor layers.
Surface emitting semiconductor light sources have several advantages over edge emitters. The light emitting surface of a surface emitter is larger than that of an edge emitter. A surface emitter needs less power to emit light than an edge emitter. Fabrication of surface emitting lasers can be less complex than fabrication of edge emitting lasers since the cleaving and mirror passivation needed for edge emitters are eliminated. A higher density of laser emitters is possible with a surface emitting source than an edge emitting source.
A surface skimming laser does use multiple active layers to emit a partial vertical beam in addition to an edge emitted beam in U.S. Pat. No. 5,319,655, commonly assigned as the present application and herein incorporated by reference. However, the laser structure has a single active layer which emits light of a first wavelength, an active layer deposited on another active layer where only the top active layer which has the shorter bandgap will emit light of a second wavelength and an active layer deposited on two other active layers where only the top active layer which has the shortest bandgap will emit light of a third wavelength. And, as noted, the light is emitted both vertically and through the edge thus dividing the optical power of the beam. A surface skimming laser also radiates vertically over an area too large to allow high density.
U.S. Pat. No. 5,319,655 also uses the surface skimming structure as a basis for a multiple wavelength vertical cavity surface emitting laser. However, each laser in this structure contains one, two or three active layers sandwiched between distributed Bragg reflectors (DBR) with no separations between the active layers or between the active layer and the upper DBR. Although the active layer in the laser with one active layer is positioned at the peak of the optical standing wave established by reflections from the DBRs, the radiating active layer in the laser containing two active layers is misaligned with that laser's optical standing wave. Similarly, the radiating active layer in the laser containing three active layers is even more misaligned with that laser's optical standing wave. Thus the surface emitting lasers taught in the patent are inefficient and have an excessively high threshold or are restricted to closely spaced wavelengths. Since poor efficiency and high threshold cause poor optical performance in laser arrays with closely spaced emitters, e.g. excessive power droop and excessive crosstalk between lasers, arrays of multiple wavelength surface emitting lasers must be designed with minimum threshold and maximum efficiency in order to be useful in optical recording applications.
It is an object of the invention to provide a monolithic structure of independently addressable, high density, multiple wavelength surface emitting lasers with broad bandwidth DBRs and improved performance.
It is another object of the invention to provide a monolithic structure of independently addressable, high density, multiple wavelength surface emitting lasers with multiple active layers for emitting light of different wavelengths.
It is yet another object of this invention to provide a monolithic structure of multiple wavelength surface emitting lasers with multiple active layers for emitting light of different wavelengths where etching removes a portion of a narrower bandgap, longer wavelength active layer to allow a wider bandgap, shorter wavelength active layer to emit light.